1. Field
One embodiment of the invention relates to an information recording/reproducing system which records sub-information by superimposing the sub-information on main information, an information transmission system which transmits sub-information by superimposing the sub-information on main information, and an information storage medium.
2. Description of the Related Art
In general, in many cases, in information recording/reproduction and information transmission/reception, in addition to main information which is composed of user data, address information for synchronism/arrangement recognition or sub-information which is used for copyright protection etc. is recorded/reproduced or transmitted. In a typical example of the method of recording/transmitting sub-information, sub-information as well as main information is encoded in such a format that the sub-information is inserted or mixed in the main information at predetermined intervals, and the main information and sub-information are transmitted by the same modulation method. In another example of the method, main information and sub-information are subjected to different encoding/modulating processes to generate recording signals or transmission signals, and then the generated signals are superimposed.
There is a case in which an encryption key for use in a copyright protection system such as CSS (Content Scramble System), or information relating to this is used as sub-information. In the copyright protection system, in order to encrypt a content that is a work and to keep an encryption key thereof secret, this encryption key is encrypted by another encryption key and the encrypted encryption key is recorded together with encrypted content, thereby preventing an illegal copy.
Such a copyright protection system effectively functions in a reproduction-only apparatus (player), but cannot prevent an illegal copy in a personal computer environment. There are violators who conduct an illegal copy by copying the whole encrypted content including the encrypted encryption key. In this situation, patent applications for copyright protection systems, in which secret information recording/reproducing systems are introduced, were filed (Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2000-3560 (paragraphs 0007 to 0009); Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2003-109302 (paragraphs 0010); and Document 3: Jpn. Pat. Appln. KOKAI Publication No. 2003-122637 (paragraph 0011)).
In these systems, secret information, on which confidentiality is required for copyright protection, is recorded by a physical modulation method that is different from the modulation method of main information, thereby making difficult reproduction and duplication of the secret information. In each of these systems, sub-information is substituted for and embedded in main information in the process of encoding (the encoding including error correction encoding and code modulation).
In each of Documents 1 to 3, the secret information modulation/recording method employs different modulation methods for the main information and the secret information in the information encoding process. Thereby, in an apparatus having only a main information reproduction system, the reproduction of secret information is disabled, and no trace of secret information remains in the processed result of the main information reproduction system. When consideration is given to the structure of the combination between an ordinary optical disc drive and a personal computer which is connected to this optical disc drive, there is an advantage that no secret information is included in data bits that are read out from the personal computer, whereby bit-by-bit copy (copy in units of a bit) can be prevented. In addition, in manufacturing optical discs storing secret information, a secret information recording system is provided to only an authorized specific disc manufacturer. On the other hand, in manufacturing an apparatus which plays back the optical disc, a secret information reproducing system is provided to only an authorized specific reproducing apparatus manufacturer. Thereby, the secret information recording/reproducing system can be disclosed to the limited, necessary range. The secret information recording/reproducing system is disclosed to only reliable manufacturers through examinations and contracts. Thus, this also serves to prevent the manufacture of illegal discs and the manufacture of illegal reproducing apparatuses by illegal manufacturers.
The superimposition of sub-information on main information means that the presence of the sub-information is a disturbance to the main information. Thus, there is a demand that the level of the sub-information be lowered to such a level that the sub-information be fully reproduced and that the reproduction of the main information may not be affected by the sub-information. However, there has been no standard for quantitatively determining a proper level of the sub-information.
In addition, since the secret information is embedded in the main information in the process of encoding, as described above, it is possible that the secret information recording/reproducing system may be decoded when a code analysis is executed by a professional. The professional, in this context, refers to, for example, an illegal disc manufacturer having manufacturing equipment and expertise which are equivalent to those of an authorized optical disc manufacturer.
The above-described problem similarly occurs in the case of signal transmission using a transmission medium in place of a storage medium, as in the case of executing recording/reproduction on the information storage medium.
As described above, when the sub-information is superimposed on the main information, a proper level of the sub-information has been unclear.
Besides, when confidential sub-information is embedded in main information, the possibility remains that the recording/reproducing system of the sub-information is decoded by a special analysis, and the sub-information is disclosed.